Radiogenic Hf Isotopic Compositions Research Articles

New discovery of the Early Paleozoic carbonatite in Bayan Obo (China): Insights into the giant REE accumulation

The Bayan Obo deposit in China hosts the world’s largest rare earth element (REE) resource. Age dating results for Bayan Obo range across approximately one billion years (from ∼1.4 to 0.3 Ga), with three age groups (∼1.3 Ga, ∼0.45–0.40 Ga, and 0.28–0.26 Ga). Carbonatite dated to around 1.3 Ga and granitoids to approximately 270 Ma have been identified. The Early Paleozoic vein-type mineralization is widespread in the deposit. However, magmatic activity related to the highest age peak at ∼0.4 Ga has not well been identified in the deposit, complicating the interpretation of the ore genesis. In this study, we report a zircon 232Th-208Pb age of 435.2 ± 4.6 Ma (MSWD = 1.9, N = 26) from carbonatite in Bayan Obo. The zircon morphology, characterized by bipyramidal crystal forms, low U concentrations (mostly < 1 ppm), high Th/U ratio, and carbonatite-like oxygen isotopes (5.92 ± 0.31 ‰), demonstrates that the zircon crystallized from carbonatitic magmas, thus their ages represent the timing of magmatic activity. Although remelting of the Mesoproterozoic carbonatite during Paleozoic carbonatite has been previously documented, the newly identified Early Paleozoic carbonatite derives from a different mantle source, as indicated by their more radiogenic Hf isotope compositions. This suggests that the Bayan Obo deposit experienced multiple carbonatitic activities. The Early Paleozoic carbonatitic activity may have facilitated element mobilization and mineral recrystallization, improving the exploitable levels of the world’s largest REE deposit.

Late Cretaceous erosion and chemical weathering record in the offshore Cape Basin: Source-to-sink system from Hf[sbnd]Nd isotopes and clay mineralogy

Source-to-sink research has often used sediment fluxes as a fundamental parameter when seeking to understand the perturbations caused by tectonics or climate. This parameter is often interpreted only in terms of erosion rates, dismissing the component exerted by chemical weathering. In this study, we characterize sediments from the Deep Surface Drilling Program core 361 (DSDP 361) located in the Cape basin to show the evolution of both, physical erosion and silicate chemical weathering, during the late Cretaceous (100.5–66 Ma) in the source-to-sink system consisting of the South African plateau and the Cape Basin. Transmission Electron Microscopy (TEM) images and trace element analyses indicate that detrital clays predominate within the site and thus reflect variations in surface continental processes. Clay mineralogy shows a predominance of detrital smectite (60–98%) throughout the section. However, an increase in primary clay minerals, in particular illite (20%), indicates a relative enhancement in physical erosion, while an increase in palygorskite (10%) indicates regional climate aridification during the Campanian–Danian interval (ca. 77–62 Ma). Concomitant to this enhancement, we observe both, more radiogenic Hf isotopic compositions and less radiogenic Nd isotopic compositions in the clay fractions (< 2 μm). These variations can be respectively associated to an increase in silicate chemical weathering and the incorporation of unradiogenic material, likely related to the orogenic belts present on the western-south margin of Southern Africa. The regional aridification and the global climate cooling trend observed on δ18O benthic foraminifera records do not appear as suitable mechanisms for the increase in physical erosion and silicate chemical weathering observed. Our results, together with the reported increase in sedimentation rates and the reported tectonic uplift between 80 and 70 Ma, suggest that it is tectonic activity and not climate the main driver in the enhancement of denudation processes. Furthermore, our data suggests that such uplift episode could have impacted the regional climate by causing a rain-shadow effect, with an inland aridification suggested by the increase in palygorskite proportions. The increase in silicate chemical weathering suggested by our new dataset begins in the Campanian (ca. 77 Ma), later than the onset of the global climatic cooling trend that occurs from the Turonian onward. If enhanced weathering of the South African Plateau may not have triggered the recorded climatic cooling, it may still have contributed to maintain cooler conditions in the Campanian-Danian interval.

Early Mesozoic magmatism records the tectonic evolution from syn- to post-collisional setting in the Central Lhasa subterrane, Tibet

The Early Mesozoic (ca. 240–185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833–857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (εHf(t) = −20.1 to −4.3). The Luoza granodiorite samples have high (87Sr/86Sr)t ratios between 0.7174 and 0.7209, and negative εNd(t) values between −9.5 and − 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (~10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher εHf(t) values (−13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (87Sr/86Sr)t ratios between 0.7062 and 0.7081, and εNd(t) values between −6.9 and − 3.4. The calculated εNd(t) values for the Menba syenogranites range from −4.5 to −4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (~50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240–210 Ma S-type-dominated to ca. 210–185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a syn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequently, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.

Forging isotopically juvenile metamorphic zircon from and within Archean TTG gneiss: Whole-rock Sr-Nd-Pb and zircon U-Pb-Hf-REE constraints

The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the protolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5–3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the LuHf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.

The nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge 33–35°N: A Sr-Nd-Hf isotopic perspective

We present new Sr-Nd-Hf isotopic data on mid-ocean ridge basalts from two ridge segments (OH-1, OH-3) between the Oceanographer and Hayes fracture zones at the Mid-Atlantic Ridge 33–35°N to constrain the nature and origin of upper mantle heterogeneity beneath the Mid-Atlantic Ridge. Together with the major and trace elements data (Niu et al., 2001), the new Sr-Nd-Hf isotopic data illustrate that the mantle sources of these lavas comprise three components, i.e., a depleted mantle with high radiogenic Hf isotopic compositions (ADM), an incompatible elemental enriched component with radiogenic Sr and un-radiogenic Nd-Hf isotopic compositions (E-type I), and an incompatible element depleted component with variably enriched Sr-Nd-Hf isotope compositions (i.e., E-type II). The ADM and E-type I components may be understood as representing ancient mantle melting residues and metasomatic veins developed at the base of the thickening oceanic lithosphere, respectively. The unique E-type II component is best explained as recent mantle melting residues of a geochemically enriched mantle, probably associated with the Azores mantle plume to the north. Taking together, we interpret the E-MORB dominated OH-1 lavas as resulting from a relatively higher degree melting of a mantle source composed of ADM matrix and E-type I material, whereas the N-MORB dominated OH-3 lavas resulting from lower degree melting of the mantle source primarily comprising ADM and E-type II components. Such a petrological and geochemical understanding explains the contrast in crustal thickness, ridge morphology and tomography, and mantle Bouguer anomalies between the two ridge segments.

Petrology, geochemistry, stratigraphy, zircon U–Pb geochronology and Hf isotopic compositions of subsurface lithologies in northwestern Mesa Central, Durango, Mexico: Implications for the tectonomagmatic evolution of northwestern Mexico

We present whole-rock geochemistry, LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions of subsurface lithologies in the northwestern corner of Mesa Central, Mexico. Zircon U–Pb ages identify five magmatic events: (1) ca. 162 Ma Nazas volcano-sedimentary province; (2) 88–61 Ma early stage Laramide age magmatism; (3) 57–48 Ma late stage Laramide age rhyodacitic/dacitic magmatism; (4) 46–41 Ma transition period andesitic magmatism; and (5) 32–28 Ma silicic volcanism of the Sierra Madre Occidental silicic large igneous province. A ca. 162 Ma zircon U–Pb crystallization age obtained for a dacitic flow interbedded within the Guanacevi conglomerate yielded generally negative εHf (t) composition varying from −3.3 to +0.8 suggesting magmas derived from crustal source(s). The new geochronological data, lithological features, and stratigraphic position indicate deposition of the conglomerate was synchronous with intermittent volcanism of the Nazas volcano-sedimentary province. Late stage Laramide age (ca. 51 Ma) rhyodacites/dacites yielded largely positive and heterogeneous εHf (t) compositions ranging from −1.6 to +6.1 indicating the dominant role of mantle-derived sources. These ca. 51 Ma rhyodacites/dacites have radiogenic Hf isotopic compositions comparable to those of Late Cretaceous – Early Tertiary Laramide plutons of the southern granite domain suggesting that they share similar subcontinental mantle sources. The 46–41 Ma andesite samples yielded dominantly negative and heterogeneous εHf (t) compositions varying from −4.8 to +4.0. Zircon U–Pb ages, and geochemical and Hf isotopic compositions indicate a rapid shift (~ 2 Ma) from predominantly mantle-derived magmas during the 57–48 Ma late stage Laramide age magmatism to more crustal dominated sources during the 46–41 Ma transition period magmatism. Therefore, we propose the development of a tear in the subducting slab underneath central Durango. The proposed tear in the subducting slab would have facilitated asthenospheric upwelling, providing the necessary heat to induce crustal melting during the 46–41 Ma transition period magmatism.

Hafnium isotope constraints on the nature of the mantle beneath the Southern Lau basin (SW Pacific)

New Hf isotope data provide new insights into the nature of the mantle beneath the southern Lau basin, adding new constraints on the displacement process of the Pacific mid-ocean ridge basalt (MORB)-type mantle by the Indian MORB-type mantle. The Hf isotopic ratios (176Hf/177Hf) of submarine lavas from the eastern Lau spreading center (ELSC) range from 0.283194 (εHf = 14.92) to 0.283212 (εHf = 15.54), with an average value of 0.283199 (εHf = 15.11) whereas those from the Valu Fa ridge (VFR) vary from 0.283221 (εHf = 15.88) to 0.283200 (εHf = 15.14), with an average of 0.283214 (15.61), indicating that ELSC lavas have a slightly more radiogenic Hf isotopic composition than VFR lavas. In contrast to the results from previous studies, the new Hf analyses combined with previous Nd isotope data clearly show that both VFR and ELSC have the distinct Hf–Nd isotope composition of the so-called DUPAL isotopic anomaly in the Indian MORB-type mantle. The DUPAL isotopic signature at VFR demonstrates for the first time that the inflow of the Indian MORB-type mantle has reached the southern tip of tectonic propagation in the southern Lau basin.

A clearer view of crustal evolution: U-Pb, Sm-Nd, and Lu-Hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern China

AbstractMuch of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6–2.3, 2.0–1.8, and 0.38–0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9–1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history—key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24–0.20 Ga metamorphism. The 0.60–0.25 Ga rutile U-Pb ages along with 0.33–0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9–2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2–1.8 and 1.5–1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9–2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone.

Grenville-age continental arc magmatism and crustal evolution in central Dronning Maud Land (East Antarctica): Zircon geochronological and Hf–O isotopic evidence

This study focusses on the Grenville-age Maud Belt in Dronning Maud Land (DML), East Antarctica, which was located at the margin of the Proto-Kalahari Craton during the assembly of Rodinia. We present new U–Pb zircon ages and Hf–O isotope analyses of mafic and granitic gneisses exposed in the Orvin-Wohlthat Mountains and Gjelsvikfjella, central DML (cDML). The geochronological data indicate continuous magmatic activity from 1160 to 1070 Ma which culminated at 1110–1090 Ma, followed by high-grade metamorphism between 1080 and 1030 Ma. The majority of zircons from the Orvin-Wohlthat Mountains exhibit radiogenic Hf isotopic compositions corresponding to suprachondritic εHf (t) values and Mesoproterozoic model ages, indicating crystallization from predominantly juvenile magmas. However, the involvement of ancient sedimentary material, which were most likely derived from the adjacent Proto-Kalahari Craton, is revealed by a few samples with negative to neutral εHf (t) and significantly elevated δ18O values (8–10‰). Samples from further west, in Gjelsvikfjella have more mantle-like zircon O isotopic compositions and late Paleoproterozoic Hf model ages, indicating the incorporation of ancient, previously mantle-derived continental crust. The rocks in cDML, thus define part of an extensive Mesoproterozoic magmatic arc with subduction under the Proto-Kalahari margin. This involved significant growth of new continental crust, possibly related to slab retreat, accompanied by subordinate recycling of older crustal components. The Maud Belt has previously been correlated with the 1250–1030 Ma Natal Belt in southern Africa, which lay to the west in the context of Gondwana, although this assertion has recently been questioned. Our study supports the latter view in demonstrating that the continental arc magmatism in the Maud Belt appears to be temporally and tectonically unconnected to the accretion of (slightly older) juvenile oceanic islands in the Natal Belt, which, in contrast to the Maud Belt, show subduction polarity away from the craton. We thus speculate that the Namaqua-Natal to Maud Belt contact (exposed in the Heimefront Shear Zone) may represent a changed tectonic environment from arc/continent-continent collision to slightly younger continental margin orogenesis at the westernmost termination of this part of the global Grenville Orogen. The Maud Belt marks the beginning of a major, long-lived accretionary Andean-type tectonic regime on the eastern margin of Proto-Kalahari in the Meso-Neoproterozoic during Rodinia assembly and break-up until the formation of Gondwana.

Lu-Hf isotopic evidence of a deep mantle plume source for the ∼2.06 Ga Bushveld Large Igneous Province

The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton at ∼2.06 Ga. In this paper we use zircon and bulk-rock Lu-Hf isotope data to determine whether the B-LIP formed in response to the arrival of a plume(s) from the deep mantle or by melting of the depleted upper mantle during foundering of an eclogitized residue at the base of the lithosphere. New zircon Hf isotope compositions for four B-LIP bodies yield intrusion-specific average εHf(2.06Ga) values that range from −20.7 ± 2.8 to −2.7 ± 2.8, largely consistent with literature zircon data for other B-LIP intrusions. Bulk-rock solution εHf(2.06Ga) values for a variety of B-LIP intrusions range from −2.1 ± 0.2 to −10.6 ± 0.2. Because the most radiogenic Hf isotope compositions across the entire B-LIP are nearly primordial, having an εHf(2.06Ga) close to 0, it is likely that the heat source of the B-LIP was a plume(s) from deep mantle. The Hf isotope data further suggests that individual intrusions in the B-LIP were produced by melting of three distinct source reservoirs (in addition to melts derived from the plume itself): 1) Subduction and plume modified continental lithospheric mantle; 2) Older (∼2.7 Ga) mafic-ultramafic plume-related material trapped in the Kaapvaal lithosphere; and 3) Mid- to upper Kaapvaal crust. The presence of ∼2.7 Ga mafic-ultramafic material in the Kaapvaal lithosphere may have acted to strengthen the lithosphere so that it was able to resist being dispersed by the arrival of the B-LIP plume at ∼2.06 Ga.

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone, eastern China

AbstractPartial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.

Sub‐mantle δ18O Zircons in Malani rhyolites: Clues for a Rodinia Linkage between South China and NW India

AbstractThe Malani Igneous Suite (MIS) in NW India represents one of the largest and well‐preserved Precambrian felsic igneous provinces, with minor mafic volcanics and dykes. The SIMS (Secondary Ion Mass Spectrometric) zircon U‐Pb geochronology yielded 776.8 ± 4.5 to 758.5 ± 6.9 Ma ages for rhyolites from Jodhpur region and Sindreth Basin while dacite sample from Punagarh Basin was dated to 760.5 ± 10 Ma. Zircons from rhyolitic and dacitic lavas have oxygen isotopic compositions that can be grouped into low δ18OV‐SMOW (4.12 to ‐1.11‰) and high (δ18O = 8.23‐5.12‰) categoroes, respectively. The low δ18O zircons have highly radiogenic Hf isotopic compositions (εHf(t)= +13.0 to +3.6) suggesting high temperature bulk cannibalization of upper level juvenile crust as the essential process for magma generation. Older than 800 Ma xenocrystic zircons in dacite have high δ18O values whereas 795 Ma ones have mantle‐like Hf‐O isotopic compositions, reflecting a significant shift in tectono‐thermal regime in NW India during 800‐780 Ma. A synchronous transition in the South China Block and Madagascar suggests a spatially and temporally linked geodynamic system. Geochemical data in combination with the new isotopic results point towards an overall convergent plate margin setting undergoing localized lithospheric extension. The NW India and South China blocks together with Madagascar and the Seychelles lay either along the periphery of Rodinia or off the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.

Origin of a Miocene alkaline–carbonatite complex in the Dunkeldik area of Pamir, Tajikistan: Petrology, geochemistry, LA–ICP–MS zircon U–Pb dating, and Hf isotope analysis

Intrusive carbonatite complexes host the world’s largest rare-earth element (REE) deposits. However, the genesis of carbonatite complexes remains unclear. A recently discovered carbonatite–syenite complex in the Dunkeldik area of Pamir, Tajikistan, is interpreted as a carbonatite-related REE deposit. The deposit is hosted in an alkaline–carbonatite belt comprising alkali basalt, alkali trachyte, syenogranite, syenitic porphyry, and carbonatite. The present study focuses on a 1–3-m-wide, 25-m-long, NE-trending carbonatite dike and associated syenitic porphyries. The carbonatite comprises mainly calcite with minor apatite, bastnäsite, allanite, and zircon. Relatively high proportions of anhydrite, gypsum, fluorite, quartz, and barite post-date the formation of the carbonatite–syenite complex. The syenitic porphyries contain phenocrysts of microcline, microperthite, and fluorite ± nepheline or quartz in a feldspathic groundmass.Carbonatite samples are characterized by low SiO2 (<5 wt%), FeO (<1.84 wt%), and MgO (<0.25 wt%), and relatively high CaO (29.39–35.04 wt%) concentrations, distinguishing them from primary magnesiocarbonatites. The associated syenitic porphyries yield high SiO2 (54.26–69.59 wt%), Al2O3 (12.73–16.08 wt%), and alkali (K2O + Na2O = 10.84–13.48 wt%; K2O > Na2O) concentrations, and low MgO (0.14–0.97 wt%) and TiO2 (0.21–0.50 wt%) concentrations. The carbonatite and syenitic porphyries show similar mantle-normalized incompatible-trace-element trends, characterized by negative gradients, extreme large-ion lithophile element (e.g., Sr and Ba) and light REE enrichment, and depletion in high-field-strength elements (e.g., Nb, Ta, P, Zr, Hf, and Ti). Laser-ablation–inductively coupled plasma–mass spectrometry U–Pb dating of zircon from carbonatite and syenite porphyry samples yields weighted mean ages of 10.66 ± 0.23 Ma (2σ) and 10.76 ± 0.09 Ma (2σ), respectively. Zircon grains in the carbonatite have moderately radiogenic Hf isotope compositions (εHf = −7.9 to +0.9). Zircons from carbonatite and syenitic porphyry samples show similar REE compositional trends, characterized by enrichment in heavy REEs relative to light REEs. Zircon grains from carbonatite and syenitic porphyry yield Th/U ratios of 0.77–1.18 and 0.85–1.62, respectively. We therefore interpret zircon grains in the carbonatite as xenocrysts inherited from the associated syenites.The similar emplacement ages and trace-element compositions of the spatially associated syenites and carbonatite suggest an origin involving liquid immiscibility. The geochemistry and Hf isotope data indicate that the magmas were derived from thickened lower crust that received an input of enriched mantle-sourced material hybridized by fluids derived from an ancient subducted oceanic slab. We infer that the carbonatite and syenitic porphyries formed in a post-collisional setting during the Miocene. The upwelling of an asthenospheric mantle diapir during the Cenozoic induced the partial melting of metasomatized lithospheric mantle and lower crust beneath the western margin of the Indian–Asian collision zone. The large-scale Karakorum strike-slip fault may have acted as a conduit for the emplacement of the syenite–carbonatite magma.

Role of ancient, ultra-depleted mantle in Mid-Ocean-Ridge magmatism

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets.

Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater

Abstract It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled eNd2.60Ga-eHf2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive eNd2.60Ga values, but still reveal decoupled eNd-eHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of eHf-eNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial eNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.

Do Hf isotopes in magmatic zircons represent those of their host rocks?

Lu–Hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution, assuming that zircon Hf isotopes can represent initial Hf isotopes of their parental whole rock. However, this assumption may not always be valid. Disequilibrium partial melting of continental crust would preferentially melt out non-zircon minerals with high time-integrated Lu/Hf ratios and generate partial melts with Hf isotope compositions that are more radiogenic than those of its magma source. Dissolution experiments (with hotplate, bomb and sintering procedures) of zircon-bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic Hf isotope compositions than fully dissolved samples. A case study from the Neoproterozoic Jiuling batholith in southern China shows that about half of the investigated samples show decoupled Hf isotopes between zircons and the bulk rocks. This decoupling could reflect complex and prolonged magmatic processes, such as crustal assimilation, magma mixing, and disequilibrium melting, which are consistent with the wide temperature spectrum from ∼630 °C to ∼900 °C by Ti-in-zircon thermometer. We suggest that magmatic zircons may only record the Hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their Hf isotopic compositions can represent the primary Hf isotopic compositions of the bulk magmas. In this regard, using zircon Hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts.

Long‐Term Geochemical and Geodynamic Segmentation of the Paleo‐Pacific Margin of Gondwana: Insight From the Antarctic and Adjacent Sectors

AbstractCombined zircon geochemistry and geochronology of Mesozoic volcaniclastic sediments of the central Transantarctic Mountains, Antarctica, yield a comprehensive record of both the timing and geochemical evolution of the magmatic arc along the Antarctic sector of the paleo‐Pacific margin of Gondwana. Zircon age populations at 266–183 Ma, 367–328 Ma, and 550–490 Ma correspond to episodic arc activity from the Ediacaran to the Jurassic. Zircon trace element geochemistry indicates a temporal shift from granitoid‐dominated source(s) during Ediacaran to Early Ordovician times to mafic sources in the Devonian through Early Jurassic. Zircon initial εHf shifts to more radiogenic Hf isotope compositions following the Ross Orogeny and is inferred to reflect juvenile crustal growth within an extensional arc system during progressive slab rollback. These new ages and Hf isotopic record are similar to those from the Australian sector, indicating that these regions constituted an ~3,000 km laterally continuous extensional arc from at least the Carboniferous to the Permian. Conversely, the South American sector records enriched zircon Hf isotopic compositions and compressional/advancing arc tectonics during the same time period. Our new data constrain the location of this profound along‐arc geochemical and geodynamic “switch” to the vicinity of the Thurston Island block of West Antarctica.

Low‐δ18O Rhyolites From the Malani Igneous Suite: A Positive Test for South China and NW India Linkage in Rodinia

AbstractThe Malani igneous suite (MIS) in NW India represents one of the best preserved silicic large igneous provinces. Voluminous silicic lavas of the MIS erupted between ~780–750 Ma. Zircon grains from rhyolite and dacite lavas have oxygen isotopic compositions that include depleted (δ18O = 4.12 to −1.11‰) and enriched (δ18O = 8.23–5.12‰) signatures. The low‐δ18O zircon grains have highly radiogenic Hf isotopic compositions (ƐHf(t) = +13.0 to +3.6), suggesting high‐temperature bulk cannibalization of upper level juvenile mafic crust as an essential mechanism to produce the low‐δ18O felsic magma. Xenocrystic zircon grains in dacites have high δ18O and low ƐHf(t) values for magmas older than 800 Ma, reflecting a dramatic transition in tectono‐thermal regime in NW India during 800–780 Ma. A synchronous transition also occurred in south China and Madagascar, suggesting a spatially linked geodynamic system. NW India and south China together with Madagascar and the Seychelles lay either along the periphery of Rodinia or outboards of the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.

The origin of highly radiogenic Hf isotope compositions in 3.33 Ga Commondale komatiite lavas (South Africa)

Drill core samples from the 3.33Ga high-alumina-type Commondale komatiites from the southern Kaapvaal craton (South Africa) were analysed for major, trace elements and Hf- isotope compositions in order to place possible constraints on their mantle sources and the process and timing of mantle depletion. The initial εHf isotope values range from +6.5 to +9.1 indicating a strongly depleted mantle source. This contrasts with previously published initial εNd values on the same geologic unit that reflect only minor depleted mantle sources of +2. Trace element modelling and modelling of the Hf-Nd isotope composition of the mantle source indicate that the strongly incompatible element-depleted trace element patterns may be explained by a mantle source that was depleted to high-degrees (ca. 40%) in the garnet stability field prior to high- degree melting of the restitic mantle in the spinel stability field. The depletion in the garnet-stability field may have been related to a plume event that produced the low-alumina Barberton-type komatiites. However, the decoupled Hf-Nd isotope compositions cannot be explained by such a mechanism. We discuss several possible scenarios that may explain the trace element distribution as well as the isotope composition of the Commondale komatiites. Based on trace and isotope modelling, we consider three different models (1) the decoupled Hf-Nd isotope compositions resulted from a depleted mantle source overprint by melt-depleted garnet-pyroxenite components that originated from older crustal differentiation events and were delaminated into the mantle; (2) the decoupling resulted from subduction fluids that overprinted a highly depleted mantle source and added Th, Nd and other light-rare-Earth elements (LREE) shortly before eruption of the komatiites and triggered the high degree of melting; (3) a possible origin from a mantle source that experienced fractionation processes in an early magma ocean, followed by at least three mantle depletion events. The trace element and Hf-Nd isotope composition of the Commondale komatiites reveal that parts of the Archean mantle must have been highly heterogeneous due to komatiite extraction and possible delamination processes of lower crustal restites.

Environmental Hf–Nd isotopic decoupling in World river clays

The hafnium and neodymium radiogenic isotope systems behave differently during Earth surface processes, causing a wide dispersion of Hf and Nd isotopic compositions in sediments and other sedimentary rocks. The decoupling between Hf and Nd isotopes in sediments is generally attributed to a combination of preferential sorting of zircon during sediment transport and incongruent weathering processes on continents. In this study, we analysed size-fractions of sediment samples collected near the mouth of 53 rivers worldwide to better understand the factors controlling the distribution of Hf and Nd isotopes in sediments. Our results for rivers draining old cratonic areas and volcanic provinces demonstrate that both granite and basalt weathering can lead to significant grain-size dependent Hf isotopic variability. While silt-size fractions mainly plot along the Terrestrial Array, World river clays are systematically shifted towards more radiogenic Hf isotopic compositions, defining together with published data a new Clay Array (εHf=0.78×εNd+5.23). The Hf–Nd isotope decoupling observed in volcanogenic sediments is best explained by selective alteration of Lu-rich mineral phases (e.g. olivine) and preferential enrichment of resistant unradiogenic minerals, such as spinel and ilmenite, in silt fractions. We also show that the extent to which World river clays deviate from the Clay Array (ΔεHf clay) is not linked to the presence of zircons. Instead, it correlates positively with weathering indices and climatic parameters (temperature, rainfall) of the corresponding drainage basins. Overall, these findings demonstrate that the distribution of Hf–Nd isotopes in clay-size sediments is related to a large extent to weathering conditions on continents, although the precise mechanisms controlling this relationship remain unclear. We finally propose that the Hf–Nd isotope pair proxy could be used in palaeoenvironmental studies to provide semi-quantitative information on past climates.